Top Global Markets, an integrated monthly report from leading providers of consumer and retail information in the video games industry, The NPD Group, GfK Chart-Track Limited and Enterbrain, Inc, has revealed that the racing game sold 8.94 million units last year.
Some 1.94 million of these sales came in the UK.
In all, Nintendo dominated the findings, with Wii Fit finishing in second spot thanks to sales figures of 8.31 million worldwide.
Grand Theft Auto IV sold 7.29 million units to take third spot, while fourth spot went to Super Smash Bros. Brawl, despite the game only registering 404,000 sales in the UK.
Call of Duty: World at War rounded off the top five.
When playing Mario Kart Wii, gamers can choose from 24 characters, which are separated into three different weight classes.
The weight of the characters determines the types of karts and motorbikes that they are able to drive.